You can win more battles because of a clever army composition than because of micro management. You only can do this when you know what roster the enemy's faction has.


THINK BEFORE YOU FIGHT

There are different components, almost every army needs. Basically the most simple are:
a) HOLDING POWER
Standard Hoplite for example have a solid holding power, but small killing power.
b) KILLING POWER.
A roman legion has a pretty good killing power, and some medium holding power too.

You need both! Almost any combination of HOLDING and KILLING can work.
Sometimes a certain unit can fulfill both roles acceptably, and the rest of the army's purpose is to support and defend.


GETTING STARTED

Rome (Polybian)

Start of with the infantry core.
A first cheap battle line is always a good choice. So lets throw in FIVE HASTATI.
Wait - what is their use? Weaken more expensive enemy units, push Skirmishers away ,throw some javelins or soak up missile damage to protect your expensive units.
But why not bring only two? Or better bring 10?
You need to find a good balance. If you bring only two, you cant use them as a first battle line. Instead you could use them to fill gaps and support your fighting principes.
If you bring ten hastati, you have so many of them in one or two lines, that they cant fight effectively.

Good. But my hastati won't be able to hold the line forever. We need some sturdy roman infantry.
Lets throw in FIVE PRINCIPES. They have high armor and high attack - good combination to finish the weakened enemy. Hastati weakened the enemy on a super cost-effective way, and the damaged and tired enemy units face your very strong principes, this works great.

Now we have two lines of melee infantry. Cavalry always adds a flexible part to your army. NEVER NEVER play without a single cavalry unit. Not bringing cavalry means that you can't get rid of enemy skirmishers effectively and you have to protect yours the whole time. Roman cavalry is pretty mediocre, THREE LEGIONARY CAVALRY should do their job.
Their use should be: Hunting skirmishers, closing gaps between your troops, catching enemy cavalry or to reinforce a fight that might not go in the right direction.

Choose safety and bring FOUR TRIARII to protect our back. When the enemy cavalry is dead, they can pretty effectively reinforce into some melee fights.
A bit of money left, a good army is always a flexible one!
TWO NUMIDIAN CAVALRY would be a bit risky but can be very useful.

Since we are a bit on this safety thing today, lets bring some foot skirmishers. We have sufficient funds, so we have to choose between Archers and Slingers. Archers deal a good amount of damage quickly, but slingers would be perfect to slowly kill some pikemen. Both can do well, it depends on the enemy. Since pikes usually are outplayed in melee too, we are going to choose TWO SYRIAN ARCHERS. Their purpose is to weaken enemy units or to possibly deal with inferior enemy skirmishers. Athens has some Javelin-Cavalry, too.

Since we have left some talents, we're going to upgrade some units. Boost the cavalry and some hastati.units TO PUT CHEVRONS ON: Cheap and medium infantry units, some mediocre cavalry. If your army points available are small, DO NOT TO PUT CHEVRONS ON elite units of any kind.

Application Note: (Constatine the Great): Good example for building a Camillian Roman Army is the Rise of Rome DLC. Good examples for building Roman Polybian Armies are the Macedon and Hannibal DLC's armies.


Athens

Build a good hammer and anvil! Use Heavy Hoplites as holding force. Use Hippeus Lancers as Killing force. Buy Tarantine Cavalry to harass enemy horses or chase skirmishers. Utilize Militia Hoplite to soak up missile damage and protect skirmishers. Then add standard Archers to weaken enemy units.